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Measurement in quantum systems is an inherently challenging problem. The lines are blurred between object, environment, and measurement apparatus, since all are governed by the same physical laws. Interactions between these three govern the results of measurements—in many cases, environmental influences dominate, an effect known as decoherence. It's an unpleasant quantum fact of life, but that doesn't mean physicists have to give up hope.

A new paper describes how to use environmental measurements to gain information about a quantum system that would otherwise be unavailable. K. W. Murch, S. J. Weber, C. Macklin, and I. Siddiqi controlled a superconducting quantum system called a transmon by performing measurements on the cavity in which it resided. In that way, they were able to monitor the transition between quantum states in the transmon without directly interacting with it. This experiment demonstrates an effective way to get around decoherence in at least some systems, which could be significant for quantum computing.

A quantum system is defined by its state: energy, position, momentum, and other physical properties. Physicists interact with these systems through various experimental equipment, which are themselves made up of particles governed by quantum mechanics. The surrounding environment is also composed of the same types of things. As a result, a measurement is an interaction between two quantum systems, and any interactions between an object and its environment can act like measurements, too.

This is important because quantum mechanics predicts the probability of outcomes of measurements. A system interacting with the random fluctuations of its environment won't remain in a stable configuration. If a physicist carefully prepares an atom or other object for experiments, quantum physics practically guarantees it won't remain in that state indefinitely; at some point it will succumb to decoherence.

Decoherence vs. persistence

The ASCII code for capital "T" is 1010100, where each digit is a single binary bit: 0 or 1. Ordinary digital electronics don't suffer from decoherence, but if they did, maybe after a certain amount of time, interaction with the environment would flip one or more of those bits randomly. So maybe "T" becomes 100 0101, which is "E." Just a few bit flips like that could render a message incomprehensible. (Think of it like magnetic storage, where uncontrolled exposure to a magnet can do something similar.) The havoc is even greater in a qubit, since the state isn't simply 0 or 1, but something more complex.

That's not a major problem for many experiments, but it becomes a headache for quantum logic, computing, and networking. A quantum bit—a qubit—needs to remain in a particular state until it can be read out, changed, or is no longer required. If decoherence strikes, that qubit is useless (see the sidebar for an analogy). The evolution of a system from a defined ("pure") state to a random decoherent state is called its "quantum trajectory," since it's akin to the path of a particle in space, though the quantum system may not actually move.

The system in the current paper involves a quantum system called a transmon, formed by confining bound pairs of superconducting electrons in a small region of material separated from a larger "reservoir" of superconducting aluminum. By exchanging particles between this region and the reservoir, the state of the transmon can be controlled precisely, allowing it to be used as a qubit. Transmons are relatively stable to decohrence, lasting for microseconds (which is pretty good for a qubit).

In the new experiment, the researchers placed the transmon in a copper microwave cavity, a chamber with reflective walls somewhat like a microwave oven. The frequency of the microwaves was such that it prevented absorption by the superconductor, so it was a bit unlike the cooking apparatus. But any microwave photons that entered the cavity could interact with the microwaves as they scattered off it.

This provided a randomly fluctuating environment for the qubit, and as the transmon interacted with the microwaves, it would induce a shift in the strength and phase (relative wave position) of the photons as they scattered. The change in photon properties depended on the transmon state, such that a measurement of the environment—the microwaves—also constituted a weak measurement on the transmon.

As a matter of fact, it's a very weak measurement, because the random fluctuations in the microwaves are much larger than the signal produced through their interaction with the transmon. That required collecting a large number of photons over time, gradually reducing the noise and revealing the evolution of the qubit in time. In that way, the researchers measured the quantum trajectory more or less in real time as a coherent pure state emerged—something known as the "collapse of the quantum wavefunction." A few trajectories even revealed a reversal, in which the system evolved toward a coherent state and subsequently decohered.

The results of this experiment point toward a way to mitigate some of the effects of decoherence. Monitoring the environment provides an additional way to measure the system, so it should be possible to spot decoherence in action. While this obviously doesn't eliminate the problem, it's a promising step toward better quantum logic systems. It also points toward new experiments in weak measurement of quantum trajectories, probing the interface between quantum and macroscopic systems.